For the detection of gamma- and X-rays, in particular in CT, dual-energy CT, SPECT and PET systems, the facilities used include direct conversion detectors based on semiconducting materials, such as CdTe, CdZnTe, CdZnTeSe, CdTeSe, CdMnTe, InP, TIBr2, HgI2. However, with these materials a polarization effect arises, particularly in the case of the high radiation flux density which is necessary for CT devices.
Polarization is the term used for the reduction in the detected count rate when there are high photon or radiation fluxes, as applicable. This polarization is caused by the very low mobility of the charge-carriers, above all at the sites of electron vacancies, or holes, and by the concentration of intrinsic imperfections in the semiconductor. The polarization thus arises from the reduction in the electric field due to fixed-location charges which are associated with imperfections, the so-called spatial charge of the semiconductor, which act as capture and recombination centers for the charge carriers produced by the X-rays. This reduces the life and mobility of the charge carriers, which in turn leads to a reduction in the detected count rate at the high radiation flux densities.
The spatial charge in the semiconductor can be unevenly distributed in the material, due to inhomogenity of the X-rays incident on the semiconductor, and can change over the course of the irradiation. A consequence of these changes is a lateral displacement of the counted events which are detected in the pixelated electrode. That is to say, the count rate of neighboring pixels are different, causing the spatial assignment of the counted events to be incorrect. Ultimately, the result is image artifacts.
Usually, the inhomogeneities in the irradiation of the semiconductor are caused by the object under investigation. In particular, the X-rays have different directions of incidence for the individual beams, due to their different scattering in the object under investigation. Consequently it is not possible to predict exactly the direction from which the X-rays strike the semiconductor. Typically however, use is made of scattered radiation grids, which absorb the X-rays which have been scattered through the object under investigation, and thereby homogenize the X-rays incident on the semiconductor. Here, the position of the scattered radiation grid is fixed relative to the semiconductor, so that the spatial inhomogeneity in the X-rays caused by the scattered radiation grid, and hence also the spatial charge, is known, because no radiation strikes in the areas directly beneath the scattered radiation grid.
Further inhomogeneities in the spatial charge can be caused by the metalized pixelated electrode attached onto the semiconductor. In the non-metalized areas of the semiconductor, that is the areas of the semiconductor which are not covered by a pixel, the electric field is weaker, and under X-ray irradiation a higher spatial charge forms.
Further inhomogeneities in the spatial charge are caused by imperfections in the material which are, however, unevenly distributed. Since their occurrence in the semiconductor cannot be controlled in terms of their spatial arrangement and frequency, it would be necessary to measure separately in each individual detector the effects they cause, in order to take these inhomogeneities into account.
Until now there has been no known solution as to how to compensate the entire inhomogeneity in the spatial charge in the semiconductor.